Because of its characteristics such as ferroelectricity, piezoelectricity, pyroelectricity and electrooptical effect, thin ferroelectric films are known in many electronic fields. In recent years, its application to memory cell in DRAM has drawn attention along with the rapid development of circuit integration. Among these ferroelectric materials, barium strontium titanate has a higher dielectric constant than strontium titanate. It can vary Curie temperature with the composition ratio of barium and strontium and thus can be used as a dielectric material depending on the working temperature of devices. Thus, barium strontium titanate is particularly favorable material. The preparation of a thin oxide film has heretofore been accomplished by a dry process such as sputtering process and vacuum metallizing process or a wet process such as sol-gel process.
However, the dry process is disadvantageous in that it requires a very expensive apparatus. It is also disadvantageous in that it requires different vapor pressures with different elements, making it impossible to effect a stable production of a thin film excellent in stoichiometry. This results in the deterioration of crystallizability. Further, the productivity is lowered, raising the production cost. Thus, the dry process is far from practicable.
On the other hand, if the wet process is used, a sol-gel process employing an organic metal compound is advantageous in accurate control of chemical composition, uniformity in molecular level, reduction of process temperature, applicability to large area, reduction of apparatus cost, etc. However, the conventional sol-gel process is disadvantageous in that it requires the use of a lower alcohol such as methanol and ethanol as a solvent, making the organic metal compound solution extremely unstable. The organic metal compound solution can absorb water content in the atmosphere to undergo hydrolysis, making it difficult to prepare a homogeneous thin film. Further, since these lower alcohols exhibit a high vapor pressure, a thin film formed by, e.g., spin coating process is dried too fast to make a thin film having a uniform thickness. In recent years, J. Am. Ceram. Soc., 71, (5), C-280 (1988), etc. have proposed the use of ethylene glycol monomethyl ether in the preparation of a thin film of a ferroelectric material such as lead titanate. However, this approach is disadvantageous in that if this solvent is used for the synthesis of a thin film of an oxide represented by formula (Ba.sub.x Sr.sub.1-x)TiO.sub.3 (0.ltoreq.x.ltoreq.1), an organic metal compound of Ba and an organic metal compound of Ti undergo selective reaction, causing precipitation. Moreover, Advanced Ceramic Materials, 3, (2), 183 (1988), etc. propose to mix a solution of barium acetate in an aqueous solution of acetic acid with titanium isopropoxide. However, this approach is disadvantageous in that titanium isopropoxide and acetic acid react with each other, possibly causing precipitation. Further, in order to accomplish the accurate control and uniformity of composition, it is necessary to synthesize a composite alkoxide having a well-controlled metallic atom ratio as a precursor. However, since water is added to the starting material, the resulting precursor solution is unstable, making it difficult to control the chemical composition of the precursor.